This invention claims priority of a German filed patent application DE P 198 53 588.0.
This invention relates to a device to hold a substrate inside an exposure system in which the substrate is seated on a table traveling in the X, Y coordinates and where, between table surface and substrate, metering means for adjusting the distance and for aligning the substrate in relation to an exposure optics are provided, from where a particle radiation is directed rectangular to the substrate surface, corresponding to the Z coordinate.
Holding devices for mounting substrates, especially for masks and wafers during exposure in optical, as well as in electro-optical exposure systems, are known to exist in different versions in the state of the art. As a rule, holding systems are arranged on a table movable in two coordinates X, Y, and have a supporting plane for the substrate on which it is seated and held prior to the exposure process while the table will be gradually moved parallel to X and/or Y, thus being adjusted to the requested exposure positions in succession. The bearing planes are mostly formed by high plane bearing surfaces, and also partially by multiple point-shaped bearing elements.
The basic bodies, mounting plates etc, on which the bearing elements are arranged or bearing surfaces are formed, are normally mechanically fixed to the table, using metering means to adjust the substrate in the Z coordinate. In this context the Z coordinate corresponds to the radiation direction of the exposure ray path and is directed rectangular to the substrate surface.
The positioning accuracy of the substrate surface, the adjustment of the bearing plane, the flatness of a bearing plane and last but not least the stability of shape of all parts and assemblies of the holding system are of essential importance for the quality and fineness gaining the structure demanded during exposure. This is the more important the more the industry of microelectronics aims at further reducing the width of structure.
For this reason an appropriate design of the holding systems have to secure accuracy in respect to dimensional and shape stability even under the influence of changes in temperature and pressure during the process of exposure. Another factor to be paid attention to with regard to the design of holding systems is, that the exposure radiation will not be affected by magnetic fields originating from magnetic components or from materials containing magnetic particles. Additionally the exposure radiation must not be deviated unintentionally by electrical chargings of the holding system. Moreover, mechanical forces, no matter of what origin, must not influence the substrate via parts or assemblies, which could cause deformations and thus inaccuracies. Besides, the costs for producing the holding device have to be kept in economically reasonable limits.
The holding systems available so far in the state of the art have to be evaluated under this very aspect of these high requirements. For example, a device for holding substrates is known from the U.S. Pat. No. 5,535,090 as well as from the publication xe2x80x9cSemiconducter Internationalxe2x80x9d, Sherman, Volume 20, No. 8, p. 319-322, having an electrostatic Chuck arrangement. Chuck arrangements have electrically conductive layers to which, through supply contacts, disconnectable electrical potentials can be applied in relation to the substrates. The application generates an electrostatic field by which the substrate is kept electrostatically on an even insulation layer placed above the conductive layer. In this context the amount of the force of attraction between the Chuck arrangement and the substrate depends on the electrical voltage fed, the area size of the conductive layer (the so-called Chuck electrode) and the thickness of the insulation layer between the conductive layer and substrate.
In case of the arrangement mentioned above, sapphire is provided to be the bearing material for the substrate. Here the whole 8xe2x80x3 inch bearing surface was not covered by sapphire but through an intermediate layer made of niobium, only several 2 inch sapphire discs are provided which form the bearing surface for the substrate. The lavish manufacturing process of the bearing surface consisting of multiple sapphire areas is disadvantageous in this context, which, in addition to the expensive sapphire material itself, causes high cost.
The U.S. Pat. No. 5,600,530 describes another holding device for substrates, which again is equipped with an electrostatic Chuck arrangement. But here aluminum oxide is used as material for the insulation layer. At the same time a procedure is mentioned by which the aluminum oxide layer is brought to the necessary thickness for positioning the substrate by thinning down.
The use of aluminum oxide implies disadvantageous problems caused by an unfavorable temperature expansion coefficient. For this reason the use of aluminum oxide inevitably requires measures to compensate this disadvantage and to prevent changes in position and/or shape of the substrate exceeding a permissible measure. The solution to this problem, however, is not given in the paper mentioned.
Another essential disadvantage of the known state of the art is that holding systems for substrates are always only designed for substrates of a given size. For the exposure of individual substrates or series of substrates of different seizes the known holding systems can be used to a certain extent only or not at all, unless under the condition of a high expenditure of assembly and adjustment.
Another holding device for substrates for processing in an electron-beam facility is revealed in U.S. Pat. No. 5,644,137. This arrangement is equipped with interferometers to determine and monitor the position of the table and the substrate when moving the X and Y coordinates. In this case, a stabilization of the position of the substrate in relation to the exposure optics is reached insofar as some parts of the holding device and the interferometers are made of material having the same expansion behavior, resulting in a higher degree of positioning precision in the X and Y directions. This publication, however, does not show how the problems, with regard to the expansion in the Z coordinate and the thus related inaccuracies, can be solved.
Taking this into consideration the invention is intended for further improvement of a holding device of the kind, described above in such a way that bearing elements for the substrate are interchangeable with each other in a quick and uncomplicated manner with yet reaching a high degree of positioning precision.
According to the invention this task is solved by two mounting plates adjusted on the table parallel to the X, Y plane, being also directed to the exposure optics and in various distances to the table surface. The first one of the mounting plates is being connected directly to the table, the second mounting plate being in connection with the first mounting plate through at least one holding device, the holding function of which can be switched on and off, with a mounting plane for substrates being designed on that side turned to the exposure optics.
This implies the advantage that the second mounting plate provided with the bearing plane for the substrate can be detached from the first mounting plate after switching off the holding function, enabling in an easy and uncomplicated way an exchange among mounting plates with bearing planes for substrates having different sizes. Due to the parallel alignment of both mounting plates to each other and the design of reference surfaces at both mounting plates and the holding devices, an exact exposure position for the substrate is secured after every exchange.
The preferred designs of the invention provide that the first mounting plate is connected to the table through anti-vibration elements, and that the switchable holding devices, designed to hold the second mounting plate to the first, are also designed to be spacer blocks through which the mounting plates are separated by a given distance parallel to the Z coordinate, creating a space between the two mounting plates. For example, a lifting device connected to a robot arm can be inserted into this space so that the second mounting plate can be taken out or exchanged for another mounting plate having a different size of the bearing plane.
According to the arrangement of the invention an advantageous construction can be reached through which different reference planes can be assigned to the individual mounting plates, thus at the same time creating an essential precondition to enable the arrangement of separate holding devices respectively parts on each mounting plate and these holding devices respectively parts are subjected to specific requirements with regard to stability in dimension and material features, so that even under extreme influences caused by temperature, pressure or mechanical forces a highly precise exposure of the substrate is secured.
For example one design of the invention can provide that at least the second mounting plate, on which the bearing plane for the substrate is formed, with regard to its material features, its dimension and its shaping is designed in such a way that forces caused e.g. by material expansions due to changes in temperature or mechanical shocks being of such an extent that they could cause a deformation of the substrate exceeding a permissible measure, can not be transferred to the substrate. In this respect the invention provides such a brittle material for manufacturing the mounting plates that does not allow a plastic deformation caused by the influence of forces.
This surely guarantees that the substrate cannot deform neither before nor during the exposure process to a degree that would imply inaccuracies during exposure.
According to the invention the connection of the first mounting plate to the table can be realized by spacer elements which on one hand are firmly mechanically connected to the table surface, and on the other hand by elastic spacers which may consist of a fluorine elastomer that are coupled to the second mounting plate.
An especially preferred design of the invention provides electrostatic Chuck arrangements as holding devices between the two mounting plates. These consist each of a basic body made of an electrically non-conductive material on which an electrically conductive layer, for instance nickel or chromium, followed by an insulation layer above, is arranged. The electrically conductive layer can be advantageously divided in respect of its plane extension into individual segments with each segment being possible to be connectable separately to an electrical potential.
Here, the surfaces of the insulation layer turned to the exposure optics are designed as bearing surfaces for the second mounting plate. Besides the holding function for the second mounting plate, these Chuck arrangements also function to form a reference plane to align the second mounting plate and thus the substrate which is arranged upon the second mounting plate as depicted.
In one design of the invented holding device the bearing plane for the substrate consists of three balls that are held on the second mounting plate by cages for instance, or are glued to the second mounting plate. These balls may be arranged on a graduated circle radially-symmetric, forming a three-point bearing for the substrate. As an alternative it can, however, also be provided that there are one or more electrostatic Chuck arrangements arranged on the second mounting plate (in addition to the Chuck arrangements serving to hold the second mounting plate) where a bearing plane for the substrate is modeled, designed to hold the substrate to the second mounting plate electrostatically. Here the basic body is connected to the second mounting plate, for instance by wringing or gluing.
In case of all Chuck arrangements the electrically conductive layer has contacts. To generate the electrostatic forces, an electrical potential is fed to the electrically conductive layer of the respective Chuck arrangement on one hand and to the assembly (the second mounting plate) put on the insulation layer respectively to the part laid on (substrate) on the other hand.
An especially preferred design of the invention provides that the second mounting plate and also the basic body and insulation layers of the Chuck arrangements are made of a magnet-free glass-ceramics with a temperature expansion coefficientxe2x80x94xcex1T=0xc2x10,05*10xe2x88x926Kxe2x88x921xe2x80x94and an elastic module of E≈90,6 GPa. Such a glass-ceramics is available on the market under the name xe2x80x9cZERODURxe2x80x9d.
The use of this material enables a construction that is extremely insensitive to changes in temperature. Influences of temperature inside the exposure system can hardly be avoided during the exposure process; they may result from thermal conduction and/or thermal radiation from within the system, but can also be effected by exposing the substrate. The construction suggested, however, allows reducing the negative implications of disturbing influences of temperature to such a degree that the expenditure spent on stabilizing the temperature within the exposure system respectively near the substrates can be reduced to a minimum.
Another advantage is that the glass-ceramics used can be processed effectively by conventional optical processing technologies securing the highest level of dimensional stability. This especially concerns the production of even planes, but also the keeping of parallelisms and angles. This thus enables the keeping of process tolerances in the micrometer range and in the range of angular seconds. Due to the brittleness of the glass-ceramics, plastic deformations on the even planes are prevented. As described above, this avoids an uncontrolled deformation of the substrate.
Another essential advantage is that traditional technologies can be used to deposit coatings onto parts made of glass-ceramics. Such coatings may be reflecting layers, for instance for interferometer mirrors, or electrically conductive layers serving to feed an electrical potential. The latter serves to avoid electrostatic charge on undesirable spots which otherwise might occur when during the exposure electrically charged particles hit insulating surfaces. To this layer the electrical potential to generate the electrostatic Chuck supporting forces between the first and the second mounting plate can be fed.
Since the glass-ceramics used is magnet-free, it is secured that the exposure beam path cannot be affected by magnetic fields, which may originate from parts made of metallic materials or from materials interspersed by magnetic particles.
To generate the electrostatic forces an electrical potential of up to 5,000 Volts is provided. The potential is switchable by means of which the supporting forces can be switched on and off as required.
It is planned for a very preferred design of the invention to have three Chuck arrangements, each having cylindrical basic bodies fixed to the first mounting plate, forming a mutual bearing surface for the second mounting plate. For example, the Chuck arrangements can be positioned radially symmetric to each other on a graduated circle. Their dimensions in direction to the Z coordinate and their distances to each other in the X and Y coordinates are designed in such a way that there is enough space left between the Chuck arrangements and between the first and second mounting plates to insert a lifting tool arrangements and between the first and second mounting plates to insert a lifting tool connected to a robot arm, to grab under the second mounting plate, to lift it off the bearing surface, to take it from the exposure system, and to replace it with a mounting plate designed to receive a substrate of a differing size.
Additionally the holding device for the substrate described can be equipped with a metrology system serving to determine and to monitor the position of the substrate in the x and y coordinates during exposure and having two mirrors adjusted to each other orthogonally serving as reference for an interferometric measuring set-up. According to the invention both mirrors can be placed either directly upon the first mounting plate or upon intermediate locks that are connected to the first mounting plate. In case of the latter, the intermediate locks serve as spacers to adjust the positions of the mirrors in direction to the Z coordinate.
In this case, too, the connection between mounting plate, intermediate locks and basic body of the mirrors can be effected by wringing or gluing. This is also advantageous, if the mirrors are made of the same glass-ceramics as mentioned above, resulting in a uniform expansion behavior of the respective parts or assemblies by temperature influence. To realize the mirror surfaces, coatings with a high reflection factor can be deposited onto the basic bodies of the mirrors, for instance made of aluminum coated with an oxide protective layer.
With regard to the mirrors it is, of course, quite possible to think of a design where the mirrors adjusted orthogonal to each other are modeled in L-figure in single piece mode and are also either directly fastened to the first mounting plate or through intermediate locks.